In the manufacture of semiconductors it is frequently necessary to supply precise volumes of different fluids, say gases, to a treatment chamber, the fluids being conveyed to the treatment chamber in sequence through a fluid feed line controlled by a diaphragm valve. Typically, a first fluid is applied to the treatment chamber through the feed line after which a purge fluid is applied to the feed line to purge any traces of the first fluid. After purging, the second fluid is then applied to the chamber through the feed line.
During the purging operation it is desirable to remove as much as possible of the first fluid in the feed line and valve so that the first fluid will not contaminate the second fluid. To accomplish this, the valve interior should have no gaps, crevices or `dead air spaces` within the chamber where fluid may be caught or trapped so that it is not purged from the valve during the purging operation. As the degree of integration of semiconductors increases, there is a corresponding demand for increased fluid purity and only a very slight amount of fluid trapped in a crevice has a serious effect on the quality of the semiconductor product, and increases the frequency of defective pieces.
In metal diaphragm valves, a crevice which traps fluid and prevents complete purging is frequently found where the metal diaphragm is mounted on a seat holder which extends through the diaphragm.
FIGS. 6 and 7 illustrate a seal structure as disclosed in U.S. Pat. No. 4,750,709 for sealing the inner peripheral edge of a diaphragm in a metal diaphragm type valve to a seat holder. The end portion of the inner peripheral edge of a diaphragm 40 is welded by a weld 80 to a eat holder 78, and an inner peripheral edge portion of the diaphragm, spaced from the weld 80 by distance D, is gripped and supported between the lower surface of a presser 82 and the upper surface of the seat holder 78 at a region 94, thereby preventing a bending stress from being applied to the weld 80 at the time of operation of the diaphragm.
In FIGS. 6 and 7, reference numeral 100 is a body, 100a is a valve chamber, 100b is a valve seat, 100c is a fluid passage, 50 is a bonnet, 160 is a seat, 88 is a stem, 86 is a screw formed at the lower end of the stem and 94 is a gripping region.
The valve disclosed in U.S. Pat. No. 4,750,709, and shown in FIGS. 6 and 7 has several disadvantages. Since the diaphragm 40 is gripped at region 94 spaced by a distance D from weld zone 80, and is supported between the lower surface of the annular presser 82 and the seat holder 78 whose entire upper surface is formed to have a curvature, a deep annular gap Go is formed between the lower surface of the diaphragm and the upper surface of the seat holder. As a result, fluid is likely to be caught in the gap so that it is not easily purged during cleaning of the valve. Accordingly, in this valve the so-called replacement or purging performance of fluids is inferior.
Moreover, since a screw structure 86 is utilized to clamp the diaphragm between seat holder 78 and the presser 82, the screw tightening may be loosened over a long course of use, and in such a case the gripping and supporting of the inner peripheral edge of the diaphragm 40 may be released. As a result, up and down movement of the seat holder 78 causes a large bending stress to be applied to the weld zone 80. This may lead to the formation of cracks in the inner peripheral edge of the diaphragm.
Further, since the diaphragm 40 is gripped in the position or region 94 spaced outwardly from the weld zone 80 by the distance D, fluid does not normally flow into the gap G between the lower surface of the diaphragm 40 and the upper surface of the seat holder 78 falling within the distance D. However, repeated vertical motion during normal valve usage causes a change in the thickness of the diaphragm at the gripping portion 94 so that air-tightness of the gap G is lost and fluid enters the gap where it is caught and not easily removed. As a result, the fluid replacement or purging performance is lowered.
The valve shown in FIGS. 6 and 7 has a further disadvantage. Since the diaphragm 40 is gripped by the presser 82 and the seat holder 78 from above and below at the annular position 94, bending stress of the diaphragm is concentrated on one point of the gripping position 94 as the diaphragm moves up or down. As a result, the diaphragm cracks or is damaged in the vicinity of the gripping position as a result of metal fatigue due to repeated movements of the diaphragm over a period of time, this damage occurring earlier than in any other parts of the diaphragm. Thus, it is hard to extend the life of the diaphragm.
FIGS. 8 and 9 illustrate a metal diaphragm valve with a diaphragm seal structure as disclosed in Japanese Laid-open Patent 114265-1996. This valve includes a valve body 100, a valve chamber 100a, a valve seat 100b, fluid passages 100c, a bonnet 50, a valve seat 160, a bushing 51, a ball 52, a stem 53 and bonnet insert 54. In this arrangement a shaft 91a of a seat holder 91 is inserted through a mounting hole 93a of a diaphragm 93 and a center opening in a weld metal member 90.
The inner peripheral edge of the diaphragm 93 is disposed between an annular flat portion 90a formed on the lower surface of the weld metal member 90 and an annular flat portion 91b formed on the upper surface of the seat holder 91. The end portion of the inner peripheral edge of the diaphragm and the portions of the member 90 and the seat holder abutting the end portions of the inner peripheral edge of the diaphragm 93 are welded together at a weld zone W around the entire circumference of the inner peripheral edge of the diaphragm thereby integrally affixing the diaphragm, the seat holder and the weld metal member.
In the valve shown in FIGS. 8 and 9, bending stress is prevented from being applied to the weld zone W during operation of the diaphragm 93 because movement of the inner peripheral edge of the diaphragm immediately outward of the weld is limited or prevented by abutment of the diaphragm against the flat portion 90a of the lower surface of the weld metal member 90 and the flat portion 91b of the upper surface of the seat holder 91. This arrangement is designed to prevent the fluid being controlled from being caught in the gap between the inner peripheral edge of the diaphragm and the upper surface of the seat holder.
Since the inner peripheral edge of the diaphragm 93 lies between the annular flat portion 90a formed in the weld metal member 90 and the annular flat portion 91b formed in the seat holder 91, the gap Go formed between the lower surface of the diaphragm and the upper surface of the seat holder is smaller in depth as compared with the corresponding gap Go of the valve shown in FIGS. 6 and 7 although this difference is not obvious from the figures because of the small dimensions involved. As a result, fluid in the valve shown in FIGS. 8 and 9 is less likely to be caught in the gap Go.
Furthermore, since the inner peripheral edge of the diaphragm 93, weld metal member 90 and seat holder 91 are affixed and integrated by welding, the risk of leakage of fluid from the end portion of the inner peripheral edge of the diaphragm is much less than in the case of the valve shown in FIGS. 6 and 7 so the sealing performance is extremely high.
Also, since the inner peripheral edge of the diaphragm is confined between the flat portion 90a of the weld metal 90 and the flat portion 91b of the seat holder 91, if the screw of the shaft 91a is loosened so that the seat holder 91 slightly moves up or down, a large bending stress is not directly applied to the weld zone W of the diaphragm. Thus, cracks are less likely to be formed in the weld zone W of the diaphragm 93 as compared with the diaphragm 40 of FIG. 6.
In the valve shown in FIGS. 8 and 9, when the diaphragm 93 moves up or down, the bending stress in the diaphragm is concentrated in the region or outermost position 94 corresponding to the outward radial extent of the flat bottom surface 90a of the weld metal member 90 and the outward radial extent of the flat upper surface 91b of the seat holder 91. The flat surfaces support diaphragm 93 radially inwardly of position 94 but the diaphragm is free to bend outwardly of this position. As a result, when the diaphragm 93 is operated repeatedly, the part of the diaphragm in the vicinity of the position 94 suffers metal fatigue and cracks earlier than other parts of the diaphragm and it is hard to extend the life of the diaphragm.
In the valve shown in FIGS. 8 and 9, the spacing between the lower surface of the diaphragm 93 and the planar upper surface 91b of the seat holder 91 is extremely small, so any fluid accumulation in the gap G between these surfaces is minute. However, repeated vertical motion of the diaphragm during normal usage causes the thickness of the diaphragm to gradually decrease, and the volume of the gap G increases. This permits a greater volume of fluid to enter the small gap and the fluid replacement performance of the valve is lowered.
It should be noted that since the length dimension l (FIG. 9) of the upper flat surface portion 91b of the seat shoulder 91 is selected to be relatively large, a relatively large mass of fluid may enter the gap G and lowering of the fluid replacement performance is a more serious problem.
The valve illustrated in FIGS. 8 and 9 has a further disadvantage in that it requires a large number of parts such as ball 52, bushing 51, bonnet insert 54 and weld metal member 90. Assembly of these parts is complicated and difficult, and it requires much time and labor to complete its assembly.